This invention relates to thermoacoustic energy conversion, and, more particularly, to thermal stacks for affecting heat energy transfer in thermoacoustic energy converters. This invention was made under the Department of Defense, U.S. Navy, and with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
In thermoacoustic energy conversion, sound is converted into a temperature gradient or a temperature gradient is converted into sound. This effect has been used to construct refrigerators, heat pumps, acoustic sources, and other useful devices. See, e.g., J. C. Wheatley, et al., "The Natural Heat Engine," Los Alamos Science (1986), and G. W. Swift, "Thermoacoustic Engines," 84 J. Acoust. Soc. Am. 1185 (1988), incorporated herein by reference, which generally discuss the theory of thermoacoustic energy conversion.
At the heart of thermoacoustic energy conversion devices is a thermoacoustic stack. The stack temporarily stores entropy so that heat will be shuttled between parcels of a working fluid excited by sound. Heat exchangers on either end of the stack exchange heat between the working fluid and the external world. Prior art thermoacoustic stacks are shown in FIG. 1: stacks of sheets 10; a honeycomb-like structure 12 with arrays of approximately square, hexagonal, triangular, or round pores; and rolled sheets 14.
To an approximation, the volume of fluid at about a characteristic length from the stack equal to the thermal penetration depth, .delta..sub.K =.sqroot.2K/.rho..sub.m c.sub.p .omega., participates in the thermoacoustic effect, where K is the thermal conductivity of the fluid, .rho..sub.m is the mean density of the fluid, c.sub.p is the specific heat of the fluid, and .omega. is the angular frequency of the sound. However, the acoustic oscillations of the working fluid also result in viscous shear stresses that lead to an undesirable energy loss mechanism that occurs in the volume of fluid generally within a viscous penetration depth, .delta..sub.v =.sqroot.2.mu./.rho..sub.m .omega., where .mu. is the viscosity of the fluid. In the prior art stack geometries shown in FIGS. 1A, 1B, and 1C, the working fluid is presented with essentially a flat or concave stack surface. The thermoacoustic volume is then approximately a rectangular volume .delta..sub.K A, and the viscous volume would be approximately .delta..sub.v A, where A is the exposed surface area of the stack. Thus, the ratio of the desirable to undesirable volumes is approximately the ratio of the thermal to viscous penetration depths. Since the thermal penetration depth is typically only 1.2-1.6 times the viscous penetration depth, a sizable fraction of the fluid is undergoing viscous energy loss. This leads to lower engine efficiencies and to additional problems in removing waste heat.
Thermal conduction in the solid stack material along the acoustic wave vector direction is an additional disadvantage of prior art stack geometries. It is clear that such heat flow decreases the useful output of the thermoacoustic device, and that the heat flow is proportional to the amount of cross-sectional area, taken across the acoustic axis, that is made up of solid material. In prior art geometries, the fraction of cross-sectional area taken up by the solid portion is high, limiting the choice of stack materials to those of low thermal conductivity.
These and other problems of the prior art are addressed by the present invention and an improved stack design is presented. Accordingly, it is an object of the present invention to reduce viscous losses in the thermoacoustic stack.
It is another object of the present invention to minimize the thermal conductivity of the stack along the acoustic axis.
Yet another object of the present invention is to increase the ratio of the volume of fluid at about a thermal penetration depth to the volume of fluid within a viscous penetration depth.
Yet another object of the present invention is to increase the ratio of the volume of fluid at about a thermal penetration depth to the volume of fluid within a viscous penetration depth.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.